The present invention relates to subterranean well cementing operations, and more particularly, to cement compositions demonstrating improved corrosion resistance, and methods of using such cement compositions in subterranean formations.
Hydraulic cement compositions are commonly utilized in subterranean operations, particularly subterranean well completion and remedial operations. For example, hydraulic cement compositions are used in primary cementing operations whereby pipe strings such as casings and liners are cemented in well bores. In performing primary cementing, hydraulic cement compositions are pumped into an annular space between the walls of a well bore and the exterior surface of a pipe string disposed therein. The cement composition is permitted to set in the annular space, thereby forming an annular sheath of hardened substantially impermeable cement therein that substantially supports and positions the pipe string in the well bore and bonds the exterior surfaces of the pipe string to the walls of the well bore. Hydraulic cement compositions also are used in remedial cementing operations such as plugging highly permeable zones or fractures in well bores, plugging cracks and holes in pipe strings, and the like.
Portland cement is commonly used in subterranean cementing applications. Drawbacks may exist to using Portland cements in certain applications, however, because they are prone to corrosive attacks by carbonic acid (H2CO3). Carbonic acid may be naturally present in a subterranean formation, or it may be produced in the formation by the reaction of subterranean water and carbon dioxide (CO2), when the latter has been injected into the formation, e.g., as in a CO2-enhanced recovery operation. Carbonic acid is believed to react with calcium hydroxide that is present in Portland cement, which reaction may cause the cement to become a soft amorphous gel. This is problematic because, inter alia, it may increase the permeability of the cement. As a result, chloride and hydrogen sulfide ions, which may be present in the subterranean formation, may penetrate the cement sheath and adversely affect, or react with, the casing. The degradation of the cement can cause, inter alia, loss of support for the casing and undesirable interzonal communication of fluids.
The susceptibility of some cements, including Portland cements, to degradation by carbonic acid may be especially problematic in typical geothermal wells, which typically involve very high temperatures, pressures and carbon dioxide concentrations. In such wells, cement failures may occur in less than five years, causing the collapse of the well casing. This, in turn, may cause lost production and may necessitate expensive casing repairs.